1. Field of the Invention
The present invention relates to a ceramics heater used to heat works such as wafers, glass substrates, etc. in semiconductor manufacturing processes or the like, for example.
2. Description of the Related Art
Ceramics heaters for heating wafers are used to perform CVD (chemical vapor deposition), PVD (plasma vapor deposition), etching, etc. in semiconductor manufacturing processes. The ceramics heaters are also used in filming apparatuses for forming thin films on glass substrates.
A conventional ceramics heater, such as the one described in Jpn. Pat. Appln. No. 3011528, includes a heater plate of ceramics, a metal foil heater wire embedded in the heater plate, etc. The heater plate is formed of a sintered product of silicon nitride or aluminum nitride. The metal foil heater wire, which is formed of a high-melting metal such as tungsten, is embedded concentrically or spirally in the heater plate.
In one such conventional ceramics heater, a metal foil heater wire having a thickness of 50 μm or less, in particular, is embedded in sintered ceramics. Possibly, the ceramics heaters involve the following problems.
[Problem 1]
Since the heater wire is thin and has a small cross section, its area ratio (S1/S2) is very low. S1 is the overall surface area of the heater wire, and S2 is the area of a work bearing surface of the heater plate. In this case, the heater plate has a lot of regions that are apart from the heater wire in the diametrical direction. Therefore, the temperature change in the diametrical direction increases with distance from the center of the heater plate, as indicated by two-dot chain line L1 in FIG. 4. Thus, the conventional ceramics heater is poor in temperature uniformity.
The heater plate for heating wafers in the semiconductor manufacturing processes, in particular, requires the maintenance of uniform temperature distribution, so that unevenness in its temperature is a serious problem. If its temperature is uneven, the heater plate is subjected to a thermal stress greater than in the case where the temperature distribution is uniform, so that it may be broken.
[Problem 2]
In the process of embedding the heater wire in ceramics material powder and sintering it, its surface layers react with carbon in the ceramics material and are carbonized, so that they may possibly suffer grain boundary cracks. If the heater wire is thin, grain boundary cracks 4 inevitably advance to the interior of a heater wire 1 through a reactive layer 3 between the heater wire 1 and ceramics 2, shown in FIG. 7 or 8. The heater wire 1 shown in FIG. 7 is 25 μm thick, while the heater wire 1 shown in FIG. 8 is 50 μm thick. FIGS. 7 and 8 are sectional views based on SEM (scanning electron microscope) photographs.
If the heater wire suffers the aforesaid grain boundary cracks, its electrical resistance is caused to exceed a normal value in the process of sintering the ceramics material powder. If the electrical resistance of the heater wire becomes higher, sufficient current cannot flow, so that the temperature characteristics of the ceramics heater are adversely affected.
[Problem 3]
Since the conventional heater wire is thin and has a small cross section, its load density (Q/S1) is high. Q is the heating value of the heater wire, and S1 is the overall surface area of the wire. If the load density is high, the heater wire may possibly snap during use (or when it is supplied with current). Since the heater wire is thin, moreover, variation of its thickness causes substantial fluctuations of the heating value and exerts a bad influence upon the temperature uniformity. Possibly, furthermore, the heater wire may snap when it is embedded in the ceramics material or sintered unless it is embedded carefully.